Cryptococcosis is a serious infectious disease in the immunocompromised patient. It is reported to be the fourth most common infectious complication of AIDS. The capsule is the most important virulence factor produced by Cryptococcus neoformans, the etiologic agent of cryptococcosis. It is the interface between the yeast and its environment. One of the most important biological activities of the cryptococcal capsule is the ability to activate the alternative complement pathway, leading to deposition of opsonic fragments of C3 into and onto the capsule. Opsonization in this manner is essential for phagocytosis of the yeast by macrophages and neutrophils. Studies during the current grant period found that the cryptococcal capsule may be the most powerful activator of the alternative pathway described to date. Indeed, it appears to be the only microbial capsule with an intrinsic ability to focus alternative pathway activation. Understanding the unique complement activating properties of the cryptococcal capsule is fundamental to understanding opsonization of the yeast and the complement depletion that occurs during cryptococcal sepsis. This latter point is of particular concern as patients with AIDS continue to develop overwhelming cryptococcemia. The overall hypothesis for this proposal is that the complement activating properties of the cryptococcal capsule are a function of one or more structural components of the capsular polysaccharide that influence initiation, amplification and regulation of the alternative pathway. This question will be approached by examination of the influence of specific molecular characteristics of the capsule on individual components of the alternative pathway. The study will pursue five specific aims: 1.) identify the component of the cryptococcal capsule that mediates initiation of the alternative pathway; 2.) determine the molecular binding site for activated C3b; 3.) identify the mechanisms by which the cryptococcal capsule regulates amplification of the alternative pathway; 4.) identify the mechanisms for regulation of decay of C3b to iC3b by the cryptococcal capsule; and 5.) determine if activation of the complement system in vivo follows the paradigms that have been established for activation in vitro. Successful pursuit of these aims will provide an understanding, at a molecular level, of one of the most important and intriguing biological activities of the cryptococcal capsule. Moreover, it will provide information regarding regulation of the complement system by a medically important microorganism.